Gas-phase fluidized-bed reactor

ABSTRACT

Ethylenically unsaturated monomers are polymerized in a gas-phase fluidized-bed reactor comprising a reactor space ( 1 ) in the form of a vertical tube, a calming zone ( 2 ) adjoining the upper part of the reactor space, a circulated gas line ( 3 ), a circulated gas compressor ( 4 ), a cooling apparatus ( 5 ), a gas distributor plate ( 6 ) which forms the lower boundary of the reactor space and, if desired, a flow divider ( 7 ), wherein the gas distributor plate ( 6 ) has a plurality of gas flow orifices ( 8 ) whose outlet sides are widened conically.

[0001] The present invention relates to a gas-phase fluidized-bedreactor for polymerizing ethylenically unsaturated monomers, comprisinga reactor space (1) in the form of a vertical tube, a calming zone (2)adjoining the upper part of the reactor space, a circulated gas line(3), a circulated gas compressor (4), a cooling apparatus (5), a gasdistributor plate (6) which forms the lower boundary of the reactorspace and, if desired, a flow divider (7), wherein the gas distributorplate (6) has a plurality of gas flow orifices (8) whose outlet sidesare widened conically.

[0002] The present invention also provides a process for polymerizingethylene or copolymerizing ethylene with C₃-C₈-α-olefins in such areactor.

[0003] Gas-phase polymerization processes are today among the preferredprocesses for polymerizing ethylenically unsaturated monomers, inparticular for the polymerization of ethylene, if desired in thepresence of further unsaturated monomers. Polymerization processes influidized beds are moreover particularly economical.

[0004] Gas-phase fluidized-bed reactors for carrying out such processeshave been known for a long time. The reactors which are customary todayhave many common structural features: they comprise, inter alia, areactor space in the form of a vertical tube whose diameter increases inthe upper part. In this calming zone, there is a lower gas flow as aresult of the larger tube diameter and this limits the fluidized bedcomprising small polymer particles. In addition, these reactors have acirculated gas line in which there are installed cooling units forremoving the heat of polymerization, a compressor and, if desired,further elements such as a cyclone for removing fine polymer dust.Examples of such gas-phase fluidized-bed reactors are described, forexample, in EP-A-0 202 076, EP-A-0 549 252 and EP-A-0 697 421.

[0005] These known gas-phase fluidized-bed reactors use gas distributorconstructions configured as perforated plates, sometimes in combinationwith an upstream flow divider, to distribute the gas uniformly acrossthe entire reactor cross section. All these constructions result,between the gas outlet orifices or drilled holes, in more or lessextensive, depending on hole spacing, horizontal, planar surfaces on theupper side of the reactor bottom against which the gaseous reactionmedium flows only to a limited extent. To avoid deposits of product onthese surfaces, it is possible, as described in EP-A-0 173 261, toarrange roof-shaped deflector plates above the bottom plate in such away that the reaction medium passes over the faces of the deflectorplates and the surface of the bottom. An exclusively vertical flow ofthe reaction medium directly into the powder bed is thus not presentwith this arrangement.

[0006] However, the vertical blowing-out of the surface of the bottomappears to be important when restarting the fluidized bed, e.g. after ashutdown; in addition, roof-shaped deflector plates lead to a higherpressure drop than is necessary for producing a uniform fluidized bedand are therefore associated with unnecessary power input and energyconsumption.

[0007] Likewise horizontal swirling of the reaction medium on thesurface of the bottom is described in EP-A-0 512 147. This is achievedby gas flow orifices running at an angle to the surface of the gasdistributor, but these are technically very complicated to manufactureand therefore have to be let into the bottom plate as individual pieces.The conspicuous length of the gas channels also makes it easier for thegas distributor plate to become blocked and makes it more difficult toclean.

[0008] In documents such as EP-A-549 252, EP-A-297 794 and EP-A-509 618,attempts are made to keep the polymer powder flowing at the surface ofthe bottom by means of gas distributor plates which are folded or angledinward. Blowing-off or -out of the surface of the bottom to avoidproduct deposits is restricted in these constructions.

[0009] It is an object of the present invention to provide a gas-phasefluidized-bed reactor whose gas distributor plate is constructed suchthat the indicated disadvantages can be avoided and deposit formation atthe upper side of the reactor bottom can be reduced in a simple way.

[0010] We have found that this object is achieved by the gas-phasefluidized-bed reactor described at the outset.

[0011] We have also found a polymerization process which is carried outin the gas-phase fluidized-bed reactor of the present invention.

[0012] The gas-phase fluidized-bed reactor depicted in FIG. 1 is onlyone of numerous possible schematic arrangements. Thus, for example, thesequence of the equipment items in the circulated gas line, particularlyof the cooler and compressor, can also be reversed or further equipmentitems can be integrated into the line. Further elements such as systemsfor discharging the product and for metering-in the catalyst are notshown in FIG. 1; such elements are known to those skilled in the art andcan be integrated into the reactor in a known manner.

[0013] An essential feature of the gas-phase fluidized-bed reactor ofthe present invention is the geometry of the bottom of the reactor and,in particular, the shape of the gas flow orifices. The conically widenedstructure of these orifices effectively reduces the formation of polymerdeposits on the upper side of the bottom without using horizontallydirected flows, and at the same time makes possible a uniform, verticalintroduction of the reactor gas into the fluidized bed.

[0014] It has been found that the angle α of the conical widenings has adecisive influence on the deposit formation. In an advantageousembodiment of the gas-phase fluidized-bed reactor, the conical wideningsof the gas flow orifices have an angle α of from 20 to 40°. Angles α ofmore than 40° should be avoided because they achieve insufficientreduction of the deposit formation, while angles α of less than 20° leadto a non-optimum bottom plate thickness if, as is advantageous, theedges of the orifices abut at the upper side of the plate or at leastleave a very small planar surface. Angles α, of from 25 to 35° areparticularly advantageous. A section through the conical orifices isshown in FIG. 2.

[0015] Another important parameter for reducing deposit formation is theplanar proportion of the surface of the bottom. This is determined bythe number of gas flow orifices, by the angle α and by the geometricdistribution of the orifices over the plate. The remaining planar partof the upper side of the gas distributor plate should be as small aspossible and is advantageously less than 20% of the total area of thegas distributor plate, preferably less than 10% and particularlypreferably less than 5% of the total area. A particularly usefularrangement of the orifices in terms of minimizing the planar surface isan offset arrangement, i.e. alignment of the orifices with gaps in theadjacent row, as shown in FIG. 3.

[0016] The number and shape of the gas flow orifices is advantageouslyselected such that the pressure drop on flowing through the bottom plateis at least 30% of the pressure drop experienced by the gas mixture onflowing through the fluidized bed. In our experience, it is advisable toemploy a value of about 50%, but a minimum pressure drop through the gasdistributor of 50 mbar and a maximum pressure drop of 250 mbar, to avoidthe danger of a jet mill or the formation of polymer dust.

[0017] To achieve an appropriate pressure drop and to distribute the gasmixture uniformly in the fluidized bed and also to prevent the polymerparticles from trickling through when the compressor is switched off, ithas been found to be useful to employ gas flow orifices which have adiameter of from 2 to 5 mm at their narrowest point.

[0018] Fears that the polymer powder could trickle through the conicalholes to the underside of the bottom to an appreciable degree, e.g. whencharging the reactor or when the fluidization process is interrupted,were unfounded. The polymer powder is held on the upper side of thebottom by bridge formation in the cones if the angle α is not tooshallow (<40°).

[0019] In a preferred embodiment of the gas-phase fluidized-bed reactorof the present invention, a flow divider (7) is fitted below the gasdistributor plate.

[0020] Various flow-directing devices, as are known to those skilled inthe art, are suitable as flow dividers. Thus, for example, inclinedmetal strips can be formed into circles or concentric octagons and thesecan be arranged in such a way that there is effectively overlap betweenthe octagons placed within one another as truncated cones. The channelsformed in this way always consist of the underside of one octagon andthe upper side of the next octagon so that the reaction medium flowspast all sides of the metal strips.

[0021] Apart from the advantages mentioned, for example the avoidance ofproduct deposits on the metal surfaces of reactor bottom and flowdivider, the arrangement according to the present invention displays, asa result of optimum gas distribution, an extremely low temperaturegradient of less than 2° C. over a fluidized bed height of about 14meters (measured from 0.5 m above the bottom plate) at a temperaturedifference of more than 40° C. between fluidized bed and the fluidizinggas entering below the bottom plate.

[0022] The gas-phase fluidized-bed reactor of the present invention isparticularly suitable for carrying out a process for polymerizingethylene or copolymerizing ethylene with C₃-C₈-α-olefins at from 30 to125° C. and a pressure of from 10 to 90 bar.

[0023] The gas-phase fluidized-bed reactor of the present invention isin principle suitable for the polymerization of various ethylenicallyunsaturated monomers. Examples which may be mentioned are ethylene,propylene, 1-butene, isobutene, 1-pentene, 1-hexene, 1-heptene, 1-octeneand also higher α-olefins; furthermore, for example, dienes such asbutadiene and cyclopentadiene and cycloolefins such as cyclopentene andcyclohexene are also possibilities. The ethylenically unsaturatedmonomers can be polymerized alone or in admixture.

[0024] The circulated reactor gas is fed in at the lower end of thegas-phase fluidized-bed reactor and is taken off again at its upper end.The circulated reactor gas is usually a mixture of ethylene, a molecularweight regulator such as hydrogen and inert gases such as nitrogenand/or saturated hydrocarbons such as ethane, butane or hexane. Inaddition, the reactor gas can comprise the above-mentionedC₃-C₈-α-monoolefins.

[0025] The velocity of the reactor gas, measured as empty tube velocity,has to be sufficiently high not only to fluidize the mixed bed of smallpolymer particles which is located in the tube and serves aspolymerization zone but also to effectively remove the heat ofpolymerization.

[0026] To set constant reaction conditions, the constituents of thereactor gas can be fed into the gas-phase fluidized-bed reactor eitherdirectly or via the circulated reactor gas. In general, it is found tobe advantageous to introduce the above-mentioned C₃-C₈-α-monoolefinsdirectly into the gas-phase fluidized-bed reactor. Furthermore, it isadvantageous in the process of the present invention to introduce thecatalyst and any cocatalysts used directly into the mixed bed of smallpolymer particles. Here, it is found to be particularly advantageous toatomize the catalyst a little at a time by means of nitrogen or argondirectly into the polymer bed using the method described in DE-A-35 44915. The cocatalysts can then be sprayed into the bed using ethylene.

[0027] In order to avoid carry-over of small polymer particles from thepolymerization zone into the circulated gas system, the gas-phasefluidized-bed reactor used for the process of the present invention hasat its upper end a calming zone having an increased diameter whichreduces the velocity of the circulated gas. In general, it is advisableto reduce the velocity of the circulated gas in this calming zone tofrom one third to one sixth of the velocity of the circulated gas in thepolymerization zone.

[0028] After leaving the gas-phase fluidized-bed reactor, the circulatedreactor gas is conveyed to a circulated gas compressor and a circulatedgas cooler. The cooled and compressed circulated gas is then conveyedvia the gas distribution devices described back into the mixed bed ofthe gas-phase fluidized-bed reactor.

[0029] In the process of the present invention too, the ratios of thestarting materials, in particular the ratio of ethylene toC₃-C₈-α-monoolefins, determine the density d of the resultingcopolymers.

[0030] Furthermore, the amount of catalyst metered in determines theproduct output of the gas-phase fluidized-bed reactor.

[0031] The pressure of the reactor gas or the pressure at which the(co)polymerization is carried out is preferably from 10 to 80 bar and inparticular from 20 to 40 bar.

[0032] The reactor of the present invention is particularly advantageousfor carrying out the polymerization in the presence of condensedmonomers and/or condensed inert hydrocarbons such as hexane, sincedeposit formation resulting from condensate droplets on the surface ofthe bottom is prevented by the devices according to the presentinvention.

[0033] The (co)polymer formed in the process of the present inventioncan be discharged from the gas-phase fluidized-bed reactor in acustomary and known manner. Owing to the particular advantages of theprocess of the present invention and of the products prepared thereby,the product can be discharged to a let-down vessel simply by opening aball valve in a discharge line. Here, the pressure in the let-downvessel is kept as low as possible in order to be able to use relativelylong transport distances and to free the (co)polymers of adsorbedliquids such as residual monomers even during discharge. The(co)polymers can then be further purified in the let-down vessel, forexample by flushing with small amounts of nitrogen. The residualmonomers which are desorbed here, the flushing nitrogen and thepropelling gas entrained in the discharge of product can be passed to acustomary and known condensation step where they are separated from oneanother again, advantageously at atmospheric pressure and relatively lowtemperatures. In general, the liquid residual monomers are returneddirectly to the fluidized bed. The remaining gas mixture can becompressed in a customary and known return gas compressor and then addedback to the circulated reactor gas.

[0034] The (co)polymers in the let-down vessel can be conveyed on to adeodorization or deactivation vessel where they can be subjected to acustomary and known treatment with nitrogen and/or steam.

[0035] The gas distributor plate of the present invention, if desired incombination with a flow divider, ensures an extremely homogeneous gasdistribution and optimum mixing of the polymer bed. The (co)polymersobtained according to the present invention can therefore be obtainedfree of agglomerates and make possible technically simpler productdischarge than is the case in the known gas-phase fluidized-bedprocesses: complicated discharge locks are no longer necessary and the(co)polymers can be discharged simply by opening and closing a ballvalve having a low cross section directly into a discharge or let-downvessel against low atmospheric overpressure without producing as ballasta high proportion of gas which would subsequently have to be separatedoff, compressed with consumption of a large amount of energy andreturned to the gas-phase fluidized-bed reactor.

EXAMPLE

[0036] A fluidized-bed reactor having a diameter of 3.5 m and a heightof 14 m was fitted with a bottom plate having the following dimensions:Thickness: 50 mm Number of holes: 19,500 Lower hole diameter:  4 mmUpper hole diameter: 25 mm Orifice angle α 30°

[0037] A continuous copolymerization of ethylene and hexene was carriedout in this fluidized-bed reactor; the reaction gas had the followingcomposition: Ethylene 44.5 mol % 1-hexene  4.5 mol % Hydrogen  6.0 mol %Nitrogen 45.0 mol %

[0038] The reactor pressure was 20 bar, the polymerization temperaturewas 100° C., the gas velocity was 0.7 m/sec. A pressure drop of 130 mbarwas measured at the reactor bottom and a pressure drop of 250 mbar wasmeasured in the fluidized bed.

[0039] The copolymerization was carried out continuously for 7 days.Subsequent inspection of the reactor found no deposits on the upper sideof the bottom.

We claim:
 1. A gas-phase fluidized-bed reactor for polymerizingethylenically unsaturated monomers, comprising a reactor space (1) inthe form of a vertical tube, a calming zone (2) adjoining the upper partof the reactor space, a circulated gas line (3), a circulated gascompressor (4), a cooling apparatus (5), a gas distributor plate (6)which forms the lower boundary of the reactor space and, if desired, aflow divider (7), wherein the gas distributor plate (6) has a pluralityof gas flow orifices (8) whose outlet sides are widened conically.
 2. Agas-phase fluidized-bed reactor as claimed in claim 1, wherein theconical widening of the gas flow orifices has an angle α of from 20 to40°.
 3. A gas-phase fluidized-bed reactor as claimed in claim 1 or 2,wherein the remaining planar part of the upper side of the gasdistributor plate is less than 10% of the total area of the gasdistributor plate.
 4. A gas-phase fluidized-bed reactor as claimed inany of claims 1 to 3, wherein the gas flow orifices of the gasdistributor plate are configured such that the pressure drop on flowingthrough the bottom plate is at least 30% of the pressure dropexperienced by the gas mixture on flowing through the fluidized bed. 5.A gas-phase fluidized-bed reactor as claimed in any of claims 1 to 4,wherein the diameter of the gas flow orifices is from 2 to 5 mm at theirnarrowest point.
 6. A gas-phase fluidized-bed reactor as claimed in anyof claims 1 to 5, wherein a flow divider (7) is installed below the gasdistributor plate.
 7. A gas distributor plate (6) as set forth in any ofclaims 1 to
 5. 8. A process for polymerizing ethylene or forcopolymerizing ethylene with C₃-C₈-α-olefins at from 30 to 125° C. and apressure of from 10 to 90 bar, wherein the process is carried out in agas-phase fluidized-bed reactor as claimed in any of claims 1 to
 6. 9. Aprocess as claimed in claim 8, wherein the polymerization is carried outin the presence of condensed monomers.